1. Field of the Invention
This invention relates to an image density control apparatus that controls density of a toner image obtained from developing a latent image by toner, the latent image being formed on a photoconductor by an exposure unit. The invention also relates to an image formation apparatus having such an image density control apparatus.
2. Description of Related Art
As a solution to changes in density of toner images due to a deterioration of the photoconductor and/or varying environment conditions, image formation apparatuses using electrophotography technology (printer, facsimile apparatus, copier, etc.) often employ an image density controller that stabilizes the density at an appropriate level and controls image formation conditions such as intensity of laser beam. As an example of the image density control, a plurality of types of test patterns are used to control image formation conditions (Prior Art 1). Also, another invention proposes a control that especially focuses on line widths within an image (Prior Art 2).                Prior Art 1: Japanese Patent Laid Open H03-279971 (FIGS. 3 and 4)        Prior Art 2: Japanese Patent Laid Open 2001-80113 (FIGS. 3, 4, and 5)        
The above described image density controls using such test patterns is able to largely control the image density so that the density is stabilized at an appropriate level despite a deterioration of the photoconductor and/or varying environment conditions. However, the above controls do not satisfy the need of securing a clear and high quality image for various types of images provided.
For example, the above image density controls cannot offer a flexible control where sufficiently thick toner (high density) is required for an all black central region of the image area configured with black pixels, in order to avoid partially missed or low toner areas, while relatively low density is required for thin lines and small characters, in order to avoid over-expanded lines and distorting characters.
Further, such an image density control employing test patterns is required to calculate an appropriate light intensity when a sensor detects the density of toner image from a test pattern. Therefore, highly accurate sensor detection is needed for a successful control that utilizes the test patterns. In addition, a more accurate control is needed since the conventional method cannot provide accurate density detection, when a sensor output reaches a saturation point depending on the types of test patterns.